Modern vehicles use a variety of sensors to measure parameters needed to control the engine, transmission, fuel system, exhaust system and other vehicle parts. A number of tests must also be performed to satisfy the many state and federal vehicle emission and fuel efficiency regulations. For example, an Engine Off Natural Vacuum (EONV) test must be performed on the vehicle fuel system each time the engine is shut off. This test is intended to detect leaks in the fuel system that might allow liquid fuel or fuel vapors to escape into the atmosphere. The test is performed by automatically closing the fuel system air vent when the engine is shut off and then monitoring the fuel tank over pressure P(t) as a function of time. If the P(t) profile matches the “normal” profile stored in the vehicle's engine management system computer memory or other computer memory, then the fuel system is considered to be substantially leak free, that is, the EONV test is passed.
As may be appreciated, the pressure P(t) in the fuel tank also depends upon the ambient temperature Ta since this affects the temperature of the fuel remaining in the tank and therefore the overpressure in the vapor space in the tank as a function of time. This means that P is a function of both ambient temperature Ta and time t. Thus, P=P(Ta,t).
The simplest way of dealing with the temperature dependence would be to place an Ambient Air Temperature (AAT) sensor on the vehicle and use the observed AAT value in evaluating P(Ta,t). Most modern vehicles have a number of temperature sensors already installed, as for example, a temperature sensor to measure Intake Air Temperature (IAT) for the engine and a sensor to determine the Radiator Coolant Temperature (RCT) and so forth. But these temperature sensors are significantly influenced by their proximity to the hot engine and do not, by themselves, give reliable readings of the Ambient Air Temperature (AAT). A sensor to specifically measure AAT is not normally found on a most vehicles and it is desirable if at all possible to avoid having to add an additional temperature sensor for this purpose because of the additional cost involved.
Accordingly, a need continues to exist to be able to estimate or predict the Ambient Air Temperature (AAT) surrounding the vehicle, using other sensors already on-board for other purposes, without adding significant additional cost. Further, it is desirable that the means and method adopted for estimating or predicting the AAT be adaptable to a wide variety of vehicle and engine types. Estimated or predicted AAT values may be used for a variety of purposes in addition to the EONV test mentioned above. These and other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.